The present invention relates to switching circuits, and in particular to current protection circuits for power switches, for example, intelligent power switches which typically have a MOSFET as an output switching transistor for driving a power load, for example, a motor.
An intelligent power switch (IPS) is an integrated circuit having a semiconductor power switching device which includes circuits for protecting the power switch, for example turning the power switching device off in the case of overcurrent, overtemperature, or other overloads. FIG. 1 shows a part of a known intelligent MOSFET power switch.
The illustrated power switch includes a main power switch 10 and an additional transistor circuit 20 that may be provided integrally with the power switch 10 or as a separate component. The transistor circuit 20 passes a small fraction of the current provided by the power switch and serves as a current sense. Typically a resistor R1 is monitored to determine the amount of current passing through the switch 10 into the load. In FIG. 1, the load is illustrated as a resistive component, but it may also be a reactive component, for example, a motor.
A comparator COMP1 monitors the voltage across the resistance, compares that voltage to a reference voltage REF1, and controls the gate voltage from the gate driver DRVR to control/reduce/terminate the gate current to the power switch 10 and the additional transistor circuit 20 if the monitored current is excessive.
The illustrated prior art circuit has certain limitations. For example, in the event of a short circuit, when the power device 10 is turned ON, the gate voltage will increase until internal regulation limits the gate voltage typically to around 3 volts.
The more problematic situation occurs when the power device 10 is ON and, as shown in a graph of FIG. 1A, a short circuit occurs. The waveform OUT shows the output of the power device 10 at that moment. The nearly vertical line dropping to zero identifies the short circuit.
When the short circuit occurs, the power device 10 is fully ON and the gate voltage is approximately 5 volts. The short circuit current ISC quickly reaches a saturation current level IDESAT, which depending on the inductance, will be considerably larger than the current limit ILIMIT. This will limit the current increase. The internal current limit block including COMP1 (FIG. 1) in the IPS will take a relatively long time to reduce a gate-source voltage VGS to around 3 volts, the gate voltage limit.
It is difficult for the circuit to react quickly to the current limit without going into oscillation and it is almost impossible to have a fast loop that will be stable with any type of load. For example, for a 20 milliohm IPS the current limit will be about 50 amps and the saturation current IDESAT with the gate-source voltage VGS of 5 volts will be approximately 300 amps. Such high currents can destroy the power device.